Sour gas is a term applied to the products of natural gas wells which contain hydrogen sulfide (H.sub.2 S), or to tail gas streams from industrial sources such as the hydrodesulfurization or hydrotreating units of an oil refinery or synthetic gas manufacture; or to the untreated refinery fuel gas or wastewater stripper off-gas streams found in petroleum refineries.
Hydrogen sulfide must be removed from sour gas for environmental and safety reasons before such gases can be used or vented to the atmosphere. Usually sour gas containing H.sub.2 s is passed into an absorption unit wherein the H.sub.2 S is absorbed in a liquid. The liquid is then regenerated in a separate vessel to produce a mixture of gases at about atmospheric pressure. This mixture of gases is termed an acid gas. It is a gas containing H.sub.2 S, usualy greater than 30 volume percent, which may also contain substantial quantities of carbon dioxide and lesser amounts of water vapor, hydrocarbons, ammonia and other chemicals. Regenerator overhead gases from a fuel gas scrubbing process or a sour water stripper also may provide the H.sub.2 S feeds (70 to 95 volume percent).
A conventional process for converting H.sub.2 S in the acid gas to water vapor and elemental sulfur is a process generally known as the Claus process. This process is suitable for acid gas streams containing greater than 30 volume percent H.sub.2 S, since at lower H.sub.2 S concentrations the combustion temperatures are difficult to adequately maintain with the conventional process. It is a low pressure process involving the following net reaction: EQU 3 H.sub.2 S+1.5 O.sub.2 .fwdarw.3 H.sub.2 O+1.5 S.sub.2 ( 1)
This reaction is usually accomplished in two stages in a reaction furnace. First, a portion, usually approximately one-third, of the H.sub.2 S in the acid gas stream is reacted with air in a free-flame combustion furnace to produce H.sub.2 S and SO.sub.2 in a net ratio of 2:1. This reaction usually proceeds at temperatures from 1800.degree. to 2900.degree. F. and pressures from 20 to 30 psi, as follows: EQU H.sub.2 S+1.5 O.sub.2 .fwdarw.H.sub.2 O+SO.sub.2 ( 2)
The remaining two-thirds of feed H.sub.2 S is then reacted with the SO.sub.2, that was produced by reaction (2) in the furnace, as follows: EQU 2 H.sub.2 S+SO.sub.2 .revreaction.2 H.sub.2 O+1.5 S.sub.2 ( 3)
Reaction (2) is exothermic and irreversible. Reaction (3) is endothermic and reversible. Reaction (1) is the net reaction of reactions (2) and (3). Because of the reversible equilibrium limitation of reaction (3), the sulfur yield in the furnace is limited to about 50 to 70% depending upon the H.sub.2 S feed concentration level.
The hot gas exiting the furnace is then cooled in a waste heat boiler (.about.700.degree. F.) to generate high pressure steam and is further cooled in a sulfur condenser (260.degree.-350.degree. F.) where liquid sulfur is condensed and separated from the gas. The gas exiting the sulfur condenser is then fed to a series of two or three stages of reheat/catalytic Claus reaction/sulfur condensation in which the remainder of H.sub.2 S and SO.sub.2 is converted to sulfur and water vapor over catalyst beds of Bauxite or activated alumina according to the following reaction: EQU 2 H.sub.2 S+SO.sub.2 .revreaction.2 H.sub.2 O+3/x S.sub.x ( 4)
where X=6 to 8 @ T=500.degree.-700.degree. F. This reaction is exothermic and reversible.
The overall sulfur yield (recovery) is typically about 92-94% for a two-stage catalytic Claus reactor train and 97-98% for a three-stage train. The tail gas comprising unconverted H.sub.2 S, SO.sub.2, sulfur vapor, N.sub.2, CO.sub.2, and H.sub.2 O is either incinerated with fuel and air and then vented to atmosphere, or sent to a tail gas cleanup unit (TGCU) to reduce the sulfur emission in order to meet more stringent emission standards and to improve the overall sulfur recovery to about 99.8%. The cost of the front-end Claus combustion furnace section is only about 20% of the overall Claus plant cost, which includes the total Claus process plus a TGCU. The front-end Claus furnace section removes 50-70% of feed sulfur at 20% of the cost. The backend catalytic Claus converter train and the TGCU unit removes the remaining 30-50% at about 80% of the overall plant cost. This uneven cost distribution for sulfur recovery is an inherent problem in the present low pressure Claus process.
A basic problem with a low pressure Claus process as described above is the fact that the water vapor produced either from the combustion furnace or from subsequent catalytic Claus converters remains in the gas stream throughout the process, which seriously limits the sulfur conversion due to the reversible nature of the Claus reaction of either reaction (3) or reaction (4). This inherent low pressure limitation thus results in an incomplete sulfur recovery, and a large gas volumetric flowrate and equipment size resulting in increased capital and operating costs in the Claus plant, the tail gas cleanup unit and the incinerator.
Although the benefits of pressure on process efficiencies are known, operation of a Claus plant at elevated pressures has not been practiced commercially because of the compression cost associated with a large volume of air and to avoid liquid sulfur condensation in catalyst beds. The resultant problems are only partially resolved by using pure oxygen or O.sub.2 -enriched air as an oxidant source whereby the inert diluent N.sub.2 is eliminated or reduced thereby increasing sulfur yield by increasing the partial pressure of the reacting gases. But the inherent limitation of high sulfur conversion remains the same because the water vapor produced from Claus reactions is still not removed.
Conditions are generally maintained in the conventional Claus converters so that temperatures never decrease below the dew point of sulfur vapor, and sulfur is prevented from condensing to liquid and plugging the catalyst beds. Sulfur is condensed at low pressure and removed from the sulfur condenser, and the process gases (H.sub.2 S and SO.sub.2) are reheated, usually by some suitable in-line heater, for further catalytic stages. Production of sulfur by reaction (4) is favored by a reduction of temperature.
Some prior art sub-dew point processes, such as in U.S. Pat. Nos. 3,702,884 and 3,749,762, for removing H.sub.2 S from gas mixtures have used low temperature catalysts beds in which sulfur may be produced as a liquid. The processes are generally used in a TGCU of a Claus process for removal of the low level of sulfur compounds. These processes involve switching operation between beds to regenerate the catalyst beds wherein liquid sulfur is condensed from the Claus reaction. Again, water vapor is not condensed in the beds or in the condenser because of low pressure.
U.S. Pat. No. 2,200,928 teaches the use of a catalyst in Claus converters (248.degree. to 842.degree. F.) which absorbs some of the water formed by the Claus reaction. This will displace the equilibrium of reaction (3) to the right to improve the sulfur yield. The catalyst must be regenerated by heating and purging with dry gas to remove absorbed water.
U.S. Pat. No. 2,258,305 discloses a system of injecting air and H.sub.2 S-containing gas into an internal combustion engine and partially combusting H.sub.2 S to form a gas containing S, SO.sub.2, N.sub.2, H.sub.2 S and water. The exhaust is cooled to condense sulfur. The exhaust is further cooled to about ambient temperature to condense out water. The exhaust gas is then reheated to a temperature at which the Claus reaction takes place to form more sulfur. This process, however, suffers from the danger of solid sulfur plugging problems in the water removal step.
U.S. Pat. No. 2,298,641 teaches using an essentially dry feed gas and incorporating a drying agent in the catalyst bed to remove water. Another scheme for removing water involves the use of two catlyst converters. The feed gas containing a small amount of H.sub.2 S is mixed with O.sub.2 and is passed into the first converter. The effluent is cooled to condense sulfur and is then further cooled to remove water. The dried effluent is mixed with air and heated and passed into the second converter, and sulfur is recovered from the reaction gases. The use of a drying agent in the first scheme requires heat regeneration which is expensive. The second scheme suffers from the same problem of sulfur plugging as in U.S. Pat. No. 2,258,305.
U.S. Pat. No. 3,822,341 teaches the use of chilled water (32.degree.-75.degree. F.) to remove water in a liquid-vapor contactor. The inlet vapor sparger or pipe distributer is directly submerged in the chilled water pool and sulfur, easily solidified on the dry surface, may present plugging problems as in U.S. Pat. Nos. 2,258,305 and 2,298,641.
U.S. Pat. No. 4,426,369 teaches a Claus process under low temperatures and low water concentration conditions. The process treats a feed stream containing sulfur compounds by first converting all compounds in the stream to a single sulfur species (either to H.sub.2 S by hydrogenation with H.sub.2 or to SO.sub.2 by oxidation with O.sub.2), reducing water to below 10% by a water quench, creating a Claus reaction mixture, and then carrying out low temperature (below sulfur melting point) catalytic conversion to sulfur and additional water.
U.S. Pat. No. 4,289,990 discloses a high pressure (5 to 50 atmospheres absolute) Claus process called the Richard Sulfur Recovery Process (RSRP). The process involves introducing a compressed H.sub.2 S- and SO.sub.2 -containing stream from the Claus reaction furnace into a RSRP catalytic reactor. The gases are reacted in a catalyst bed in the reactor to produce elemental sulfur under appropriate temperature and pressure such that water in the RSRP reactor exists only as water vapor and sulfur vapor is condensed in the catalyst bed. The condensed sulfur is removed from the catalyst bed as a liquid. In this process water vapor is not condensed out with liquid sulfur so that the catalyst can remain effective and to eliminate potential corrosion resulting in the need for alloy steel in the process equipment.
U.S. Pat. No. 4,419,337 discloses another version of the RSRP process for generating SO.sub.2 and SO.sub.3 from sulfur or hydrogen sulfide by means of an oxidizing catalyst. This process replaces the conventional Claus reaction furnace with a RSRP oxidizer, which oxidizer is followed by the RSRP reactor described in U.S. Pat. No. 4,280,990. The oxidizing catalyst requires that water exist only as water vapor, and that the water not be condensed out with the liquid sulfur.
U.S. Pat. No. 4,138,473 teaches a modified Claus process by repressurizing the effluent gas stream from each sulfur condenser before entering the next catalytic Claus converter to improve the sulfur yield. In this process the condition is such that the water vapor is not condensed out with sulfur. The water vapor is condensed out only in a quench tower from the tail gas of the process in which all sulfur species are first oxidized with O.sub.2 to SO.sub.2. The dried SO.sub.2 is then recycled to the front-end Claus furnace for further sulfur conversion.
U.S. Pat. No. 4,279,882 teaches the use of a catalytic Claus process called the Selectox process wherein the conventional thermal reactor, including its combustion chamber and waste heat boiler, is replaced by a catalytic selective oxidation reactor. There is no concurrent condensation of water and sulfur.
U.S. Pat. No. 4,302,434 teaches a hydrogenating desulfurisation process which produces liquid sulfur and gaseous hydrogen, and which utilizes a recycle of the remaining H.sub.2 S process gas stream. The water vapor is condensed out only in a quench tower after hydrogenation.